Method for discriminating between ventricular and supraventricular arrhythmias

ABSTRACT

The present invention is direction toward a detection architecture for use in implantable cardiac rhythm devices. The detection architecture of the present invention provides methods and devices for discriminating between arrhythmais. Moreover, by exploiting the enhanced specificity in the origin of the identified arrhythmia, the detection architecture can better discriminate between rhythms appropriate for device therapy and those that are not.

CROSS-REFERENCE OF CO-PENDING APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/856,084, filed May 27, 2004, and now U.S. Pat. No. 7,330,757; whichclaims the benefit of U.S. Provisional Application Ser. No. 60/474,323,filed May 29, 2003; and which is also a continuation-in-part of U.S.patent application Ser. No. 09/990,510, filed Nov. 21, 2001, titledAPPARATUS AND METHOD OF ARRHYTHMIA DETECTION IN A SUBCUTANEOUSIMPLANTABLE CARDIOVERTER/DEFIBRILLATOR, and now U.S. Pat. No. 6,754,528;the disclosures of which are incorporated herein by reference.

FIELD

The present invention relates generally to a method and means fordiscriminating between cardiac rhythms appropriate for therapy using animplantable cardioverter defibrillator. More particularly, the presentinvention relates to a detection architecture having a detectionenhancement operator that discriminates between supraventriculararrhythmias and ventricular arrhythmias.

BACKGROUND

Effective, efficient systemic circulation depends on proper cardiacfunction. Proper cardiac function, in turn, relies on the synchronizedcontractions of the heart at regular intervals. When normal cardiacrhythm is initiated by the sinoatrial node, the heart is said to be insinus rhythm. However, when the heart experiences irregularities in itscoordinated contraction, due to electrophysiologic abnormalities thatare either inherited, induced, or caused by disease, the heart isdenoted to be arrhythmic. The resulting cardiac arrhythmia impairscardiac efficiency and can, be a potential life threatening event.

In a heart monitoring system it is often desirable to distinguishbetween ventricular complexes that are conducted by the intrinsicconduction system from the atria, and ventricular complexes thatoriginate in the ventricle. Cardiac arrhythmias arising from the atriaof the heart are called supraventricular tachyarrhythmias (SVTs).Cardiac arrhythmias arising from the ventricular region of the heart arecalled ventricular tachyarrhythmias (VTs). SVTs and VTs aremorphologically and physiologically distinct events. VTs take manyforms, including ventricular fibrillation and ventricular tachycardia.Ventricular fibrillation is a condition denoted by extremely rapid,nonsynchronous, and ineffective contractions of the ventricles where theventricular complexes of ventricular fibrillation arise from multiplelocations. This condition is fatal unless the heart is returned to sinusrhythm within a few minutes. Ventricular tachycardia are conditionsdenoted by a rapid heart beat in excess of 120 beats per minute, butfrequently as high as 150 to 350 beats per minute, that has its originin a single location within the ventricle. This location, which isfrequently abnormal cardiac tissue, typically results from damage to theventricular myocardium from a myocardial infarction or some other heartmuscle disease process. Ventricular tachycardia can and frequently doesdegenerate into ventricular fibrillation.

SVTs also take many forms, including atrial fibrillation, sinustachycardia and atrial flutter. These conditions are characterized byrapid contractions of the atria. Besides being hemodynamicallyinefficient, the rapid contractions of the atria can also result in anelevated ventricular rate. This occurs when the aberrant electricalimpulse in the atria are transmitted to the ventricles via the intrinsicconduction system. Although an SVT can result in significant symptomsfor the patient, it is usually not life threatening.

Transvenous implantable cardioverter/defibrillators (transvenous ICDs)have been established as an effective treatment for patients withserious ventricular tachyarrhythmias. Transvenous ICDs are able torecognize and treat tachyarrhythmias with a variety of therapies. Thesetherapies range from providing anti-tachycardia pacing or cardioversionenergy for treating ventricular tachycardia to high energy shock fortreating ventricular fibrillation. Usually, the transvenous ICD deliversthese therapies in sequence starting with anti-tachycardia pacing andthen proceeding to cardioversion (or low) energy and then, finally, highenergy shocks. Sometimes only one of these is selected depending uponthe tachyarrhythmia detected. This sequence or selection of therapy iscalled “tiered” therapy. To effectively deliver these treatments, theICD must first classify the type of tachyarrhythmia that is occurring,after which appropriate therapy is provided to the heart. A problemarises, however, when the ICD delivers therapy to what was mistakenlyclassified as a ventricular tachycardia, but was actually a highventricular rate caused and sustained by an SVT.

A major limitation of both past and present transvenous ICDs isinaccuracy in differentiating tachycardias requiring therapy, andtachycardias for which therapy is not appropriate. Inappropriateelectrical therapy from currently available commercial andinvestigational devices has been reported during documented periods ofsinus rhythm, sinus tachycardia and supraventricular tachycardiasincluding atrial flutter and atrial fibrillation.

Besides being painful, when a transvenous ICD delivers inappropriatetreatment to a patient, it is extremely disconcerting to the patient.Moreover, it can induce worse cardiac arrhythmias and even lead to adeterioration in cardiac contraction strength. Accurate discriminationof an SVT versus a potentially lethal ventricular tachycardia is,therefore, an important factor in ensuring that appropriate therapy isdelivered to an arrhythmic heart.

For the reasons stated above, and for other reasons stated below, whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forproviding a reliable system to discriminate between SVT and ventriculartachycardia and SVT and ventricular fibrillation.

SUMMARY

The detection architecture of the present invention provides methods andmeans for discriminating between arrhythmias. In exemplary embodimentsof the present invention, the detection architecture uses variousmethods to direct therapy toward the treatment of ventriculararrhythmias. The present invention compares specific attributes of asensed cardiac complex to a stored cardiac template. In particularembodiments, the stored cardiac template is updated following eachsensed beat.

The present invention may also utilize multiple templates and multiplevector views to compare specific attributes of the sensed cardiaccomplex in order to discriminate between rhythms. In particularembodiments, the present invention may capture different sensing orvector views and compare the sensed cardiac complex to its correspondingstored template.

In particular embodiments of the present invention, a series ofoperations are performed that systematically eliminate possiblearrhythmias until the identified arrhythmia is accurately classified.The classification of the arrhythmia is particularly aided by thepresent invention's ability to accurately determine the origin of theidentified arrhythmia. Additionally, by exploiting the enhancedspecificity in identifying the origin of the arrhythmia, the detectionarchitecture can better discriminate between rhythms appropriate fordevice therapy and those that are not.

Furthermore, the present invention's ability to discern particularatrial arrhythmias permits the present invention to be used in treatingparticular atrial arrhythmias, or other arrhythmias that requiretreatment, as well. For example, the detection architecture of thepresent invention may be used in devices where it is desirable todiscriminate and treat particular supraventricular tachycardias. Andlastly, as a result of the above-described improvements, the timingassociated with applying appropriate therapy may be a function of therhythm identified and the malignancy of the identified rhythm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate, respectively, representative subcutaneous andintravenous ICD systems;

FIG. 2 shows in block form an illustrative embodiment including arepresentation of how a detection enhancement operator may be engaged bya rate triggering event;

FIG. 3 shows an exemplary embodiment of a sinus template generated foruse in the present invention;

FIG. 4 illustrates the amplitude change noticed when switching betweenvector views;

FIG. 5 shows a sensed cardiac complex that correlates poorly with asinus template;

FIG. 6 shows a sensed cardiac complex that correlates well to a sinustemplate;

FIG. 7 shows a sensed cardiac complex having a narrow QRS measurement;

FIG. 8 shows a sensed cardiac complex having a wide QRS measurement;

FIG. 9 shows an exemplary embodiment of the present invention using acascade of comparison methods;

FIG. 10 depicts a sampled electrocardiogram having a normal sinussegment and a segment having an arrhythmic event;

FIG. 11 depicts a graph showing the Boolean results on the sampledelectrocardiogram using comparison method A and comparison method D;

FIG. 12 depicts a graph showing the Boolean results on the sampledelectrocardiogram using comparison method E and comparison method D;

FIG. 13 depicts a graph showing the Boolean results on the sampledelectrocardiogram using comparison methods A, D and E;

FIG. 14 through FIG. 19 illustrates other detection enhancement operatorembodiments of the present invention using cascading and the BooleanANDing of comparison methods;

FIG. 20 through FIG. 29 show additional detection enhancement operatorembodiments using cascading non-Boolean comparison methods; and

FIGS. 30 and 31 depict graphs illustrating how the present invention maybe utilized to discriminate supraventricular arrhythmias.

DETAILED DESCRIPTION OF THE DRAWINGS

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are numberedidentically. The drawings, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Those skilled in the art will recognize that many of theexamples provided have suitable alternatives that may be utilized.

The present invention is generally related to ICD systems that providetherapy for patient's experiencing particular arrhythmias. The presentinvention is directed toward detection architectures for use in cardiacrhythm devices. In particular, the present invention is suited for ICDsystems capable of detecting and defibrillating harmful arrhythmias.Although the detection architecture is intended primarily for use in animplantable medical device that provides defibrillation therapy, theinvention is also applicable to cardiac rhythm devices (includingexternal devices) directed toward anti-tachyarrhythmia (ATP) therapy,pacing, and other cardiac rhythm devices capable of performing acombination of therapies to treat rhythm disorders.

To date, ICD systems have been transvenous systems implanted generallyas shown in FIG. 1B, however, as further explained herein, the presentinvention is also adapted to function with a subcutaneous ICD system asshown in FIG. 1A.

FIG. 1A illustrates a subcutaneously placed ICD system. In thisillustrative embodiment, the heart 10 is monitored using a canister 12coupled to a lead system 14. The canister 12 may include an electrode 16thereon, while the lead system 14 connects to sensing electrodes 18, 20,and a coil electrode 22 that may serve as a shock or stimulus deliveryelectrode as well as a sensing electrode. The various electrodes definea number of sensing vectors V1, V2, V3, V4. It can be seen that eachvector provides a different vector “view” of the heart's 10 electricalactivity. The system may be implanted subcutaneously as illustrated, forexample, in U.S. Pat. Nos. 6,647,292 and 6,721,597, the disclosures ofwhich are both incorporated herein by reference. By subcutaneousplacement, it is meant that electrode placement does not requireinsertion of an electrode into a heart chamber, the heart muscle, or thepatient's vasculature.

FIG. 1B illustrates a transvenous ICD system. The heart 30 is monitoredand treated by a system including a canister 32 coupled to a lead system34 including atrial electrodes 36 and ventricular electrodes 38. Anumber of configurations for the electrodes may be used, includingplacement within the heart, adherence to the heart, or dispositionwithin the patient's vasculature. For example, Olson et al., in U.S.Pat. No. 6,731,978, illustrate electrodes disposed in each chamber ofthe heart for sensing, as well as shocking electrodes in addition to thesensing electrodes. While Olsen et al. make use of atrial event countswithin periods defined by ventricular events relying on near fieldsensing, the present invention has identified distinct methods fromthese that, in various embodiments, provide improved sensing both interms of capturing deleterious cardiac events and reducing falsepositives and unnecessary shocks.

The detection architecture of the present invention provides a methodand means for discriminating between arrhythmias. Moreover, byexploiting the enhanced specificity in the origin of the identifiedarrhythmia, the detection architecture can better discriminate betweenrhythms appropriate for device therapy and those that are not. Inexemplary embodiments of the present invention, the detectionarchitecture uses various techniques illustrated herein to directtherapy toward the treatment of ventricular arrhythmias. However, thepresent invention's ability to discern particular atrial arrhythmiaspermits the present invention to be used in treating particular atrialarrhythmias, or other arrhythmias that require treatment, as well. Forexample, the detection architecture of the present invention may be usedin devices where it is desirable to discriminate and treat particularsupraventricular tachycardias. Furthermore, the timing associated withapplying appropriate therapy may be a function of the rhythm identifiedand the malignancy of the identified rhythm.

In some embodiments, a detection enhancement operator is included. Thedetection enhancement operator may be engaged continuously, or becomeactive in response to an event or a combination of events. In certainembodiments, the detection enhancement may be engaged by identifyingparticular rhythm patterns (i.e., long-short-long intervals). Otherevents capable of triggering the detection enhancement include thepatient's cardiac rate, or discernable deviations in the cardiac rate(i.e., reduction in heart rate variability). For example, in single ormulti lead systems (whether subcutaneous, epicardial or transvenous),these rate deviations may be identified by an abruptly acceleratingventricular rate (“paroxysmal onset”). In alternative embodiments, thedetection enhancement operator may be engaged by having the rate surpassa preset or dynamically adjustable rate threshold.

FIG. 2 illustrates an embodiment 40 of the present invention showing howthe detection enhancement operator 42 may be engaged by a ratetriggering event. In the embodiment illustrated, a rate triggeringenhancement zone 44 is formed between a low rate boundary 46 and a highrate boundary 48. The low rate boundary 46 represents the ceiling forrates failing to trigger the detection enhancement operator 42.Similarly, the high rate boundary 48 represents the floor for ratesrequiring therapy. Thus, only rates between the low rate boundary 46 andthe high rate boundary 48 (the rate triggering enhancement zone 44) arecapable of triggering the detection enhancement operator 42. Ratesexceeding the high rate boundary 48 are presumptively rates that requiretherapy, and therefore, the detection enhancement is bypassed.

The “cardiac rate” may be determined by measuring the interval betweensuccessive cardiac complexes. For example, the count may be determinedby measuring the interval between successive R-waves. This example is inno way exhaustive as numerous alternative methods for calculatingcardiac rate are known to those skilled in the art.

Regardless of how the cardiac rate is monitored, high and low cardiacrate threshold boundaries may be used. Examples of these thresholds areshown in the illustrative detection enhancement scheme of FIG. 2. Inpreferred embodiments of the present invention, the low and high rateboundaries 46, 48 are programmable between 170 bpm and 260 bpm. Byadjusting the rate boundaries, the rate triggering enhancement zone 44may be as large as 90 bpm (high rate at 260 bpm and low rate at 170 bpm)or bypassed altogether (placing the low rate boundary at 260 bpm).Although the examples above illustrate the low rate boundary beingadjustable, the high rate boundary may also be adjusted. For example,the high rate boundary may be set at 240 bpm for adult patients, whereasfor children, the high rate boundary may be programmed at 280 bpm.

In the embodiment illustrated in FIG. 2, the low rate boundary 46 is setat 170 bpm and the high rate boundary 48 is set at 260 bpm. If thesensed cardiac rate is below the low rate boundary 46 (or below 170bpm), no action is taken and the detection architecture continues tosense the cardiac rate. When the cardiac rate is above the low rateboundary 46, the detection architecture then considers whether thecardiac rate is greater than the high rate boundary 48 (or above 260bpm). If the cardiac rate is greater than the high rate boundary 48,therapy is deemed appropriate and the capacitors are charged and therapyis delivered. Once therapy is delivered, the cardiac rate is againmonitored. Alternatively, if the cardiac rate is above the low rateboundary 46 and below the high rate boundary 48, the detectionarchitecture engages the detection enhancement operator 42. Thedetection enhancement operator 42 aids in discriminating betweenarrhythmias having rates falling within the programmed rate triggeringenhancement zone 44. The function of the detection enhancement operator42 is described in detail below.

Once the detection enhancement operator 42 is engaged, the portion ofthe cardiac cycle relating to ventricular depolarization is evaluatedmathematically and compared to a template. The mathematical comparisonmay be accomplished using a number of different methods (described indetail below); however, the method of comparison is generally dependenton the template used. In some embodiments, the mathematical comparisonmay include a numerical calculation that does not require a template,such as QRS width trends, R-wave width, and R-wave width variance.Further, in some embodiments a mathematical comparison may include andetermination of whether a rate accelerating event has occurred.

An ICD system of the present invention, in a preferred embodiment, iscapable of storing and using multiple templates. Templates applicable tothe detection enhancement operator may include those that are static ordynamic. Static templates are cardiac complexes that are capturedpreviously in time and stored for reference by the device.Alternatively, dynamic templates are cardiac complexes that are capturedcontinuously and compared to the subsequently detected cardiac complex,or a cardiac complex occurring some number of complexes later in time.Regardless of whether the template is static or dynamic, the templatemay be a snapshot of a single cardiac complex, or alternatively, anaveraging of previously sensed cardiac complexes.

An example of a static template is a stored sinus complex. The storedsinus template may be acquired in a number of different ways. Forexample, the stored sinus template may be a cardiac complex selected bya physician. In one embodiment, the physician may capture a cardiaccomplex observed when the implanted or applied device is incommunication with a programmer. After the physician detects arepresentative sinus complex, the physician may capture the complex onthe programmer and set this complex as the sinus template forcomparison. In an alternate embodiment, the physician may select anartificially created sinus template. This form of template is oneartificially modeled to resemble a typical sinus complex. Yet anotherexample of a static template is a stored sinus template thatautomatically updates after a preset period of time, or after a presetnumber of sensed complexes.

Static templates can also be formed from a cardiac complex that followsa triggering event. In certain embodiments, the cardiac compleximmediately following the triggering event will be stored as thetemplate and each subsequent complex will be compared to this storedtemplate. In alternative embodiments, a cardiac complex forming thetemplate is captured following a preset number of beats following thetriggering event. The number of beats between capturing the template andthe triggering event is programmable. In these embodiments, the numberof beats between the capturing of the template and the triggering eventis programmable between 2 and 14 beats. It is then this later capturedtemplate that is compared to each subsequently sensed complex.

An example of a dynamic template is a template that is continuouslyupdated after each sensed cardiac complex. Such a dynamic templateenables a mathematical comparison between the most currently sensedcardiac complex and the complex immediately preceding it. Alternatively,a dynamic template can also compare the most currently sensed cardiaccomplex to a template that represents an average of a selected number ofpreviously sensed cardiac complexes. To illustrate, if the dynamictemplate comprises an average representation of the last four sensedcomplexes, with the sensing of each complex the aggregate template willadd the newest sensed complex and discard the oldest sensed complex.Thus, with each additionally sensed complex the aggregate template isupdated.

Dynamic templates may be formed and used continuously in the presentinvention, or alternatively, they may be formed and used only followingthe observance of a triggering event. In these embodiments, once atriggering event is observed, the dynamic template is created andsubsequently updated with each cardiac complex following the triggeringevent. In certain embodiments, this dynamic template reverts back to astored sinus template following a preset number of beats, or aftertherapy is delivered. Further examples of static and dynamic templatesare described herein.

An exemplary embodiment of a sinus template (for either a static or adynamic template) generated for use in the present invention is shown inFIG. 3. The template depicted in FIG. 3 is illustrative only. Thepresent invention is not limited in terms of how the template is formedand/or the template's particular configuration.

The template 50 depicted in FIG. 3 was generated by sampling a cardiaccomplex during normal sinus rhythm. The template complex 50 comprisesthirty samples 52 of the single cardiac complex having a fixed samplingfrequency 54 between samples. The peak 56 of the sampled normal sinuscomplex is placed at the center of the template 50. From the center peak56, fifteen samples 52 are established to the left of the peak 56 andfourteen samples 52 are established to the right of the peak 56. Byaligning the template 50 with a sampled cardiac complex, mathematicalcalculations may be performed to determine how well the sampled complexcorrelates with the template 50. Because the sampling frequency 54between samples 52 is fixed, the correlation between the template 50 andthe sampled complex may be mathematically evaluated. From thesemathematical calculations the detection enhancement operator 42 maydiscern particular attributes of the sensed cardiac complex and helpdiscriminate whether the sensed complex indicates whether treatment isindicated.

It should be noted that several of the illustrative embodiments andanalyses have been prepared using a sampling rate of 256 Hertz. Thissampling rate is merely illustrative, and any suitable sampling rate maybe used (e.g., 128 Hertz). While the illustrative example of FIG. 3relies upon a collection of thirty samples around a center point, othersampling methods and numbers, as well as different “windows” may beused. Greater or fewer numbers of samples may be used (at higher orlower sampling rates, if so desired), and the peak need not be placed inthe center of the sampling window. Any signal feature that is amenableto repeatable sensing may be used to align the template with a sensedcardiac complex.

Along with utilizing templates that are stored at different times(static or dynamic), the detection enhancement operator 42 of thepresent invention may utilize templates capturing different sensing orvector views. Referring back to FIG. 1A, in this configuration, the ICDsystem can sense a plurality of vector views, V1, V2, V3, V4. Thus, theconfiguration depicted in FIG. 1A would permit at least four differentsensing views for a single cardiac complex in time. Moreover, thedetection enhancement operator is preferably capable of storing the fourvector views individually as four different templates. The invention,however, is not limited in terms of lead or electrode types, lead orelectrode configurations, or sensing templates formed from any exemplarymode or configuration. Moreover, more sensing electrodes than are shownin FIG. 1A may be added to the lead 14 and/or the canister 12 resultingin sensing vectors not described above.

The detection enhancement operator 42 of the present invention, in apreferred embodiment, can further mathematically compare acquiredcardiac complexes (or their vector representations) from two views(e.g., V1 and V2) to their corresponding stored sinus template views.This configuration enhances the detection enhancement operator's abilityto discern supraventricular based arrhythmias from ventricular basedarrhythmias. More specifically, it is extremely unlikely that aventricular based arrhythmia would appear the same as its stored sinustemplate in both views. In such an instance, at least one of the twoviews would indicate a morphology change, based on its origination inthe ventricle, when compared to the stored sinus templates. Thus,although there may not be a discriminating difference in one view (e.g.,V1) between a ventricular based arrhythmia and a stored cardiactemplate, by examining a second view (e.g., V2), the distinction wouldmore likely be pronounced.

In some preferred embodiments of the present invention, the ICD systemexamines vector views that are generally oriented orthogonally to oneanother. By using orthogonally oriented vector views, if one vector viewdetects only nominal electrical activity because of its orientation, agenerally orthogonal vector view from the first should detectsignificantly larger electrical activities. FIG. 4 demonstrates thisprinciple.

FIG. 4 illustrates twenty-three cardiac complexes. The first twelvecardiac complexes are sensed using vector view V1. Following the twelfthcardiac complex, the ICD system begins sensing using vector view V2.Thus, the remaining eleven cardiac complexes, following the pause 58,are sensed using vector view V2.

The average electrical activity for a cardiac complex using vector viewV1 in FIG. 4 is approximately 0.35 mV. In contrast, the averageelectrical activity for a cardiac complex using vector view V2 isapproximately 1.61 mV. Therefore, a nearly 360% change in sensitivitywas observed by switching between vector views. Thus, by having theability to switch between views, a vector view may be chosen thatpossesses the best signal to noise ratio for R wave detection, and hasthe best sensitivity to observe particular attributes the detectionarchitecture may use for discriminating between arrhythmias.

While orthogonal views provide the opportunity to capture a maximumamplitude in one vector view when a minimum amplitude is experienced inits generally orthogonal alternative vector view, having two viewsprecisely orthogonal to one another is not a requirement of the presentinvention. Any relative angle may be used, and the use of multiple viewsis seen as one aspect of several embodiments that may improve sensingperformance. If desired, even three or more views may be used in onecomparison or mathematical analysis.

In some embodiments of the present invention, the ICD systemcontinuously monitors its various vector views for the view possessingthe best signal to noise ratio. This may be particularly important whenthe patient changes body posture or position or during alterations inrespiration when signal amplitude may change for any particular vector.When a better vector view is observed, the ICD system switches to thisvector view and utilizes a corresponding template to monitorindividually sensed cardiac complexes. In alternative embodiments, theICD system monitors additional vector views only when the currently usedvector view experiences considerable noise or if sensing is less thanoptimal.

The detection enhancement operator 42 of the present invention mayutilize any one of the above described templates in combination. Forexample, the detection enhancement operator 42 may compare a sensedcardiac complex in vector view V1 to a stored sinus template of the samevector view. At the same time, the detection enhancement operator 42 mayadditionally compare the most recently sensed complex to the one justprevious in time in vector view V2. In this example, two vector views, astatic template and a dynamic template are used in combination. Thus,the detection enhancement operator 42 may utilize several of thetemplates in combination to more accurately determine the type ofarrhythmia and whether the arrhythmia originates from the ventricles orwhether the arrhythmia is supraventricular in origin.

The detection enhancement operator 42 performs a decision making processthat may be enhanced through morphology comparisons. The detectionenhancement operator 42, for example, may compare the morphology of asensed cardiac complex by one of many methods to one or more of thedescribed templates. The mathematical comparisons between the sensedcardiac complex and the template are performed on particular attributesof the cardiac complex. In some embodiments, the attribute of comparisonin a sensed complex is the slew rate, the polarity, the signal frequencycontent, the width of the QRS complex, the amplitude of the cardiaccomplex, or any combination of these or other distinguishablemorphological attributes. Moreover, these attributes, and others, may becorrelated to produce a reliable metric for quantifying waveformchanges. Correlation Waveform Analysis (CWA) employs the correlationcoefficient as a measure of similarity between the template and thewaveform under analysis. The correlation coefficient can be used toproduce reliable metrics to distinguish waveform changes.

FIGS. 5 and 6 illustrate one embodiment of how sensed cardiac complexescompare to a stored sinus template. The sinus templates 60 in FIGS. 5and 6 are formed as described in detail with reference to FIG. 3.Samples indicating the sinus template 60 are shown as open circlemarkers. The samples indicating the sampled cardiac complex 62, 64 areshown as cross markers. The examples of FIGS. 5 and 6 make use of thirtysamples of an individual cardiac complex; more or less samples may beused with the embodiments illustrated herein.

The sinus template 60 comprises thirty fixed length samples having apeak 66, fifteen samples to the left of the peak 66 and fourteen samplesto the right of the peak 66. The comparison technique is initiated bypositioning the peak of the sensed cardiac complex 62, 64 at thecorresponding peak reference point 66 for the sinus template 60. Thedetection enhancement operator 42 then places cross markers, for thevalues representing the sensed cardiac complex 62, 64 at the same fixedlength sampling frequency as those circle markers representing thevalues for the sinus template 60. Following this step, the detectionenhancement operator 42 mathematically compares the correlation betweenthe sinus template 60 and the sensed cardiac complex 62, 64. In oneembodiment, this comparison evaluates particular attributes that giverise to the difference between the two sets of markers. This correlationtechnique is repeated for each sensed cardiac complex.

In FIG. 5, the difference between the sensed cardiac complex 62 and thesinus template 60 is considerable. On a CWA scale of zero to 100, wherezero means minimal correlation and 100 means a perfect correlationbetween the compared waveforms, the sensed cardiac complex 62 in FIG. 5scored a zero. The sensed cardiac complex 62 in FIG. 5 thereforecorrelated poorly with the sinus template 60. Specifically twenty-one ofthe thirty cross markers for the sensed cardiac complex 62 did notoverlap the circle markers of the sinus template 60. In fact, there is aconsiderable amount of separation between the sinus template 60 markersand the markers for the sensed cardiac complex 62. Thus, the sensedcardiac complex 62 in FIG. 5 does not resemble a normal sinus cardiaccomplex.

In contrast, the sensed cardiac complex 64 in FIG. 6 scored over eightyon the same CWA scale as used in FIG. 5. In FIG. 6, only eleven of thethirty cross markers of the sensed cardiac complex 64 did not overlapthe circle marker of the sinus template 60. Moreover, the difference inseparation between those sinus template 60 markers that did not overlapthe markers for the sensed cardiac complex 64 was negligible. As such,the sensed cardiac complex 64 in FIG. 6 correlated strongly with thesinus template 60, and therefore, strongly indicates that the sensedcardiac complex 64 represents a normal sinus complex.

The detection enhancement operator 42 of the present invention is, in apreferred embodiment, capable of running real time CWA, or othermorphological analysis, on each beat. For example, each consecutivecomplex can be compared to the next one (using a dynamic template), oralternatively, each consecutive complex can be compared to the first inthe series (using a static template). This ongoing comparison techniquecan be used to determine in real time if the morphology is mostlyunchanging from complex to complex, changing somewhat from complex tocomplex, or changing significantly from complex to complex—or otherwisegenerally observing the variability behavior between complexes measuredunder CWA. Thus, along with the correlation metric derived from runninga CWA, a variability metric is gleaned from examining the variability inthe CWA from complex to subsequent complex.

Another attribute of comparison utilized to distinguish ventricular andsupraventricular events is the width of the QRS complex. Although thisexamination does not compare the QRS width of a complex to a template,it does compare the QRS complex to a predetermined width thresholdvalue. In exemplary embodiments, the QRS width value is determined bymaking a series of measurements on each individual complex. FIGS. 7 and8 illustrate how the width value is calculated and shows on twodifferent sensed cardiac complexes whether the complexes are narrow orwide.

In the illustrative example, width value is first calculated byidentifying the peak height. In one embodiment, the peak height ismeasured in ADC units. An ADC is an analog to digital converter, whichconverts analog signal (in a given range) to its digital equivalentvalue. For example, an 8 bit ADC operating in the range of +/−10 mV willconvert an analog signal of +/−10 mV into +/−127 ADC units. For example,an analog signal of 10 mV is converted to +/−127 ADC units and an analogsignal of −10 mV is converted to −127 ADC units, with linear mappingin-between. With respect to ADC, it may be noted that the use of any oneparticular format for the digital information (signed/unsigned, ones ortwos complement, etc.) is not a requirement of the present invention.

In FIG. 7, the peak height 68 for the sensed cardiac complex 70 isapproximately seventy-two ADC units. This would correspond to a 5.6 mVsignal (10 mV*72÷128=5.6 mV) going into the ADC. It should be noted thatthe initially received signal may be filtered and amplified beforereaching the ADC.

After calculating the peak height 68, the peak height 68 measurement isdivided in half (72÷2=36) to determine the width threshold value 72. Thewidth threshold value 72 for the sensed cardiac complex 70 isapproximately thirty-six ADC units, and is indicated by a dotted line.In particular embodiments of the present invention, a narrow cardiaccomplex is indicated when fewer than approximately thirty-five percentof the sampled complexes lie above the width threshold value 72, and awide cardiac complex is indicated when more than approximatelythirty-five percent of the sampled complexes lie above the widththreshold value 72.

According to the above described parameters, the sampled cardiac complex70 in FIG. 7 is narrow. This Figure shows that seven samples out of atotal of thirty samples lay above the width threshold value 72.Therefore, approximately 23 percent of the sampled cardiac complex 70lay above the width threshold value 72. Using the parameters defined,the sampled cardiac complex 70 would be labeled by the detectionenhancement operator 42 as a narrow cardiac complex.

The sampled cardiac complex 74 in FIG. 8, in contrast, is wide. FIG. 8depicts twenty out of a total of thirty samples lay above the widththreshold value 72. Therefore, approximately 67 percent of the sampledcardiac complex 74 lies above the width threshold value 72, and as such,would be considered a wide cardiac complex by the detection enhancementoperator 42.

In alternative embodiments, the QRS width threshold value is set to apreset value. For example, the width threshold value is set to 100milliseconds in particular embodiments. Thus, complexes having QRSwidths less than 100 milliseconds are considered narrow, whereas QRSwidths greater than 100 milliseconds are considered wide. By using an Xout of Y filter, a grouping of complexes may be assessed ascharacteristically wide or narrow. It is then this grouping that may beutilized by the detection enhancement operator 42, alone or incombination, to detect and discriminate between particular arrhythmias.Although 100 milliseconds is used for illustrative purposes, otherembodiments of the present invention may use QRS width values betweenapproximately 60 and approximately 175 milliseconds.

Interval rate stability, although not morphological, can also be used asan attribute for comparison. Interval rate stability measures the timingbetween subsequent complexes. In preferred embodiments, the intervalbetween a first complex and a second complex is within +/−30milliseconds of the interval between the second complex and a third,subsequent complex. In alternative embodiments the interval ratestability is between +/−5 and +/−85 milliseconds. Interval ratestability is low when deviations in the rate interval fall outside ofthe predetermined value. Again, a grouping of complexes may be assessedas having a high or low interval rate stability by using an X out of Yfilter. The grouping is then analyzed by the detection enhancementoperator 42 to discriminate between arrhythmias, the malignancy of thearrhythmias, and the appropriateness of delivery therapy.

A single event that may be used as an attribute for comparison is rateacceleration. Rate acceleration is the abrupt change of cardiac rate(typically considered “abrupt” if occurring within approximately 3-10cycles) to an elevated and sustained rate of over 120 bpm. This abruptrate change is characteristic of particular arrhythmias and itsappearance may be utilized by the detection enhancement operator 42 ofthe present invention, alone or in combination, to detect anddiscriminate between particular arrhythmias.

Utilizing the templates and the comparison techniques described indetail above, the detection enhancement operator 42 of the presentinvention may direct therapy. In preferred embodiments of the presentinvention, the detection enhancement operator 42 uses these techniquesto direct therapy toward the treatment of ventricular arrhythmias.Examples of ventricular arrhythmias that the detection enhancementoperator 42 intends to treat include monomorphic ventricular tachycardia(MVT), polymorphic ventricular tachycardia (PVT) and ventricularfibrillation (VF). These are arrhythmias that are considered malignantand therefore require therapy from an implantable device such as an ICD.Similarly, the detection enhancement operator 42 of the presentinvention works to preclude the treating of supraventriculararrhythmias. Examples of supraventricular arrhythmias include atrialfibrillation (AF), atrial tachycardia (AT) and sinus tachycardia (ST),where therapy should be avoided; when the intent is to treat aventricular tachyarrhythmia.

The present invention's ability to discern particular atrialarrhythmias, however, also permits the implementation into devicesdesigned for treating particular atrial arrhythmias, or otherarrhythmias that require treatment. For example, the detectionarchitecture of the present invention may be used in devices where it isdesirable to discriminate and treat particular supraventriculartachycardias, among others, when desired.

Referring now to Table 1, a comparison chart is depicted representingseveral comparison methods (outlined in detail below) and the predictedoutcomes of these comparisons with various arrhythmias. The arrhythmiasin Table 1 include both ventricular arrhythmias requiring therapy andsupraventricular arrhythmias where therapy should be withheld. AlthoughTable 1 describes several comparison methods to aid in discriminatingbetween arrhythmias, the present invention is not limited in terms ofthe scope of Table 1. Other comparison methods may be used, and arecontemplated, to populate a similar table for discriminating betweenarrhythmias.

TABLE 1 AF AT/ST MVT PVT VF A HIGH HIGH LOW LOW LOW B LOW LOW LOW HIGHHIGH C HIGH HIGH HIGH LOW LOW D LOW LOW LOW HIGH HIGH E NARROW NARROWWIDE WIDE WIDE F LOW HIGH HIGH LOW LOW G NO YES/NO YES YES YESTable 1 uses the following comparison methods and their correspondingdefinitions:

A=CWA between a sensed complex and a stored sinus template, where HIGHindicates high correlation with a stored sinus template and LOWindicates low correlation with a stored sinus template;

B=Variability in the CWA between a sensed complex and a stored sinustemplate, where HIGH indicates high variability within a grouping ofcardiac complexes and LOW indicates low variability within a grouping ofcardiac complexes;

C=CWA between a sensed complex and a template acquired after atriggering event (here, a template representative of a complex with arate between 170 and 260 bpm), where HIGH indicates high correlationwith the template acquired after a triggering event and LOW indicateslow correlation with the template acquired after a triggering event;

D=Variability in the CWA between a sensed complex and a templateacquired after a triggering event (here, a template representative of acomplex with a rate between 170 and 260 bpm), wherein the template isdynamic and continually updated by the previously sensed cardiaccomplex, where HIGH indicates high variability in the CWA, within agrouping of cardiac complexes when compared to a template acquired aftera triggering event and LOW indicates low variability in the CWA within agrouping of cardiac complexes when compared to a template acquired aftera triggering event;

E=Comparison to a QRS width threshold value (described in detail above),where WIDE indicates QRS waveforms having greater that 35 percent oftheir complex laying above the width threshold value and NARROWindicates QRS waveforms having less that 35 percent of their complexlaying above the width threshold value;

F=Interval rate stability of +/−30 milliseconds, where YES indicatesstability within +/−30 milliseconds and NO indicates stability outsideof +/−30 milliseconds; and

G=A rate acceleration event, where YES indicates a rate acceleratingevent and NO indicates the lack of a rate accelerating event.

For the purposes of the Table 1, a scaled CWA is considered HIGH if itexceeds 50, where the CWA is scaled to be a number between 0-100.Because the CWA is a measure of correlation, in terms of raw data theCWA could potentially have a score between −1 and +1. For Table 1, thescaled CWA is scaled such that any negative CWA result is given a zero,while positive CWA (in raw data) values are multiplied by one hundred toyield a range from 0-100 for the scaled CWA. Using this scale, a CWAbelow 50 would be considered LOW. Any suitable scale may be used, asdesired, or, the CWA may be treated directly without scaling.

For some embodiments, the definitions of HIGH and LOW for the CWA mayvary from method to method. For example, while for method A the dividingline between HIGH and LOW may be at about 50 (using the scaled CWA wherenegative coefficients are zeroed and positive coefficients aremultiplied by 100), method D may look for stronger beat-to-beatsimilarity and set the dividing line at about 70.

By extrapolating the observations in Table 1, it is observed thatcertain comparison methods may be used to discriminate treatablearrhythmias from arrhythmias where therapy should be withheld. Thisdiscrimination process can be accomplished using a single comparisonmethod, or using multiple comparison methods.

The use of a single comparison method to discriminate between all thetreatable arrhythmias and those arrhythmias where therapy should bewithheld is illustrated using comparison method A. If when runningcomparison method A the correlation was low, as denoted as LOW in thetable, then this result would indicate that the cardiac complex did notcorrelate with the stored sinus template and that the arrhythmiaresembled either MVT, PVT, or VF. In contrast, a score of HIGH in thiscomparison method indicates an arrhythmia that correlated highly withthe stored sinus template, and is indicative of AF, AT and ST in thetable. Thus, by running comparison method A alone and receiving a score,the detection enhancement operator 12 of the present invention wouldallow the delivery or withholding of therapy, depending on the devicerequirements. The other comparison method that discriminates all thearrhythmias indicating therapy from arrhythmias where therapy should bewithheld is comparison method E. In particular, a WIDE score incomparison method E would indicate the delivery of therapy for MVT, PVTand VF arrhythmias and not for AF, AT and ST.

Alternatively, some comparison methods alone can only distinguishparticular arrhythmias, and not all the arrhythmias which indicateeither therapy is required (i.e., PVT and VF, but silent on MVT) ortherapy should be withheld (i.e., AT and AF, but silent on ST). Anexample of this phenomenon is illustrated by comparison method B. If aHIGH score resulted when running comparison method B, this HIGH scoreonly distinguishes PVT and VF arrhythmias from the other arrhythmias. AHIGH score does not discriminate all of the treatable arrhythmias fromthe not-treated arrhythmias. Specifically, the treatable arrhythmia MVTscores LOW in comparison method B. A LOW score is also indicative of AF,AT and ST. Thus, alone, comparison method B cannot discriminate alltreatable arrhythmias from arrhythmias where therapy should be withheld.The other comparison methods that discriminates certain arrhythmiasindicating therapy from arrhythmias where therapy should be withheld arecomparison methods C, D and F. These comparison methods similarly onlydiscriminate PVT and VF arrhythmias from the other arrhythmias when usedsingly. Although these comparison methods may not seem ideal in somecircumstances because they do not discriminate all of the treatablearrhythmias, in particular situations, it may make good clinical senseto detect and treat only the most discordant scores.

Certain arrhythmias in Table 1 are strongly indicated when processedthrough certain comparison methods. These results are unambiguous evenwhen sensed by transvenous lead systems. An example of this phenomenonis the strong indications observed in PVT and VF arrhythmias whenrunning comparison method A. Specifically, a sensed PVT or VF arrhythmiccomplex will almost always correlate poorly (score as LOW) when comparedto a stored sinus template. The ambiguity in this comparison isextremely low with these arrhythmias. Thus, scoring a LOW in comparisonmethod A lends itself to a strong indication for these two particulararrhythmias.

Table 2 shows which of the illustrative comparison methods tease outparticular arrhythmias with little to no ambiguity, or alternatively,show a strong indication for the particular arrhythmia.

TABLE 2 AF AT/ST MVT PVT VF A — — — LOW LOW B — LOW LOW HIGH HIGH C — —— LOW LOW D — LOW LOW HIGH HIGH E — — — — — F — — — LOW LOW G — —/NO — ——

Certain entries in Table 1 are influenced by some ambiguity. BecauseTable 1 was tabulated from data observed by transvenous lead systems,these systems cannot always unambiguously discern vector informationthat distinguishes attributes specific to particular arrhythmias. Thereason for this is that transvenous electrode systems are optimized forlocal information sensing, their optimization comes at the expense offar field and vector information sensing. This relative lack of farfield and vector information sensing translates to relatively frequentambiguous sensing with certain arrhythmias, such as an atrialfibrillation that is conducted rapidly to the ventricles.

The ambiguity of certain arrhythmias can be high in particularcomparison methods. Table 3 shows which of the illustrative comparisonmethods tease out particular arrhythmias with high ambiguity and theircorresponding estimate of ambiguity percentage for a transvenousapproach. Table 3 shows the weak indicators for particular arrhythmias.Again, this ambiguity is primarily the result of the data populatingTables 1-3 being observed from transvenous lead systems.

TABLE 3 AF AT/ST MVT PVT VF A — — LOW — — (20%) B — — — — — C — — HIGH —— (20%) D — — — — — E NARROW NARROW — — — (33%) (33%) F — — — — — G NO(20%) — — YES YES (20%) (20%)

Of note, the ambiguity percentages used in Table 3, and all subsequentTables, are educated estimations based on published studies and clinicalobservations. It is believed that these results are suitable forextrapolation to a larger population. However, ambiguities in the Tablesexist. For example, it is estimated that about 20% of the populationwill contraindicate an MVT when using either comparison method A orcomparison method C. For some embodiments of the present invention,these ambiguity percentages provide a tool for planning amulti-comparison methodology. By knowing the relative ambiguities of anyparticular comparison method, the detection enhancement operator maydetermine particular comparison methods more efficaciously over otherswhen discerning particular arrhythmias.

An example of the ambiguity of certain arrhythmias when using certaincomparison methods is illustrated when examining comparison method A. Inillustration, in transvenous studies, although a MVT arrhythmia willgenerally correlate poorly (score as LOW) when compared to a storedsinus template, there is an approximately 20 percent chance that a MVTmay demonstrate a high correlation and actually score HIGH using thesame comparison method. Thus, the influence of these more ambiguousresults is troubling when discriminating between arrhythmias, andultimately in directing therapy.

To compensate for ambiguities in transvenous lead systems, or to addspecificity in determining the applicability of therapy, the comparisonmethods (A-G) may be layered as one-sided algorithms. Layeringcomparison methods permits maximum efficiency in decision making by thedetection enhancement operator. One-sided algorithms (comparisonmethods) do not necessarily identify certain arrhythmias, but thisregime can identify what types of arrhythmias a sensed complex is not.By cascading and layering one-sided algorithms, the specificityincreases and the identity of an arrhythmia may be established withgreat certainty.

This comparison technique can either be single or dual sided.Specifically, the detection enhancement operator 42 can withhold therapyonly based on comparison, can deliver therapy only based on comparison,or can hold or deliver therapy based on comparison. These combinationalmethods can also be used to make decisions as to diagnostic informationcollected, either alone or in combination with therapy. However,two-sided algorithms and the running of multiple comparison methodssimultaneously do not necessarily add specificity to the detectionenhancement operator. In illustration, if multiple comparison methodswere run simultaneously, it is possible for the results of thesimultaneous run to be worse than if only one comparison method was usedalone. This is possible because one comparison method may introduceambiguity that does not overlie the second comparison method when thetwo are run together, thereby increasing the ambiguity in the result.Moreover, if the comparison methods were set up so that one is alwaysdeferred to, then there would be no need to run the second comparisonmethod at all.

In preferred embodiments of the present invention, it is beneficial tobegin the layering of comparison methods with those that introduce theleast ambiguity. Thus, all the subsequently following comparison methodsare left only to tease out a small percentage of arrhythmias notidentified by the first, or preceding comparison methods. By cascadingthe appropriate comparison methods, the detection enhancement operatorof the present invention may properly discriminate a preponderance ofthe arrhythmias that may present themselves to an implantable device.

An exemplary model depicting a cascade of comparison methods isillustrated in FIG. 9. The detection enhancement operator 42 shown inFIG. 9 is engaged by having a rate sustained within the rate triggeringenhancement zone (RTEZ) 44. If the rate is below this RTEZ 44, thedetection enhancement operator 42 is not activated and the systemcontinues to monitor the patient's heart rate. If the rate threshold ismet, the detection enhancement operator 42 is engaged and subsequentlyevaluates the following first layer of questioning 75—does comparisonmethod A result in a LOW score and (a Boolean AND) does comparisonmethod D result in a HIGH score? A “yes” answer to this Boolean queryunambiguously identifies arrhythmias PVT and VF. Both PVT and VFdemonstrate strong indications for these queries, as illustrated inTable 2. Additionally, because these arrhythmias require therapy, thedevice would then be directed to deliver a therapeutic shock followingthis “yes” answer. A “no” answer to this Boolean query, however, wouldresult in the detection enhancement operator 42 asking a second layerquestion 77.

In the second layer of questioning 77, the detection enhancementoperator 42 evaluates the following—does comparison method D result in aHIGH score and (a Boolean AND) does comparison method E result in a WIDEscore? A “yes” answer to this Boolean query most likely identifies thearrhythmia MVT. The necessity for asking the second layer question 77 isto remove any ambiguity that a MVT arrhythmia did notuncharacteristically correlate highly in comparison method A when askedin the first layer of questioning 75. If a MVT did correlate highly (asindicated as possible in Table 3) the first layer of questioning 70could miss this arrhythmia that would require treatment. However, byBoolean ANDing comparison method D with comparison method E, thepreponderance of MVT arrhythmias would be detected. More specifically,there would be an extremely low probability that a MVT arrhythmia wouldcorrelate highly with a stored sinus template and also have a narrow QRScomplex. Thus, a “yes” answer to this Boolean query identifies thearrhythmia MVT and a “no” answer to this Boolean query identifies asupraventricular arrhythmia.

FIGS. 10-13 illustrate how the cascade described in FIG. 9 functions ona sample electrocardiogram, and further how the detection enhancementoperator 42 identifies an arrhythmia in the sample electrocardiogram.Unlike the method described in FIG. 9, however, the graphs in FIGS.11-13 are the result of the detection enhancement operator 42 beingcontinually on, and not triggered through an RTEZ 44. As such, thegraphs will include markers for both normal sinus rhythms as well asrhythms following a triggering event to illustrate the effectiveness ofthe cascading technique.

FIG. 10 is a 500 second sample electrocardiogram having a normal sinusrhythm segment 78 and an arrhythmia segment 79. The rhythm prior to theapproximately 215 second mark is indicative of normal sinus. However,following the 215 second mark the rate accelerates abruptly anddramatically.

FIG. 11 depicts graphically the first layer of questioning 75—doescomparison method A result in a LOW score and (a Boolean AND) doescomparison method D result in a HIGH score? The results of comparisonmethod A are plotted on the y-axis of the graph and the results ofcomparison method D are plotted on the x-axis. After plotting theresults of this question against all of the complexes in the sampleelectrocardiogram, three distinct regions appear.

The first region 80 in FIG. 11 is indicative of rhythms ofsupraventricular origin. More specifically, the rhythms in the firstregion 80 are normal sinus and correspond to those cardiac complexesobserved prior to the 215 second mark in the sample electrocardiogram.These rhythms have cardiac complexes that do correlate well with thenormal sinus template and would have low variability with a templateformed from a preceding complex. As such, these complexes populate thetop left hand corner of the graph.

The rhythms populating the second region 82 and the third region 84 ofthe graph are ventricular in origin and indicative of ventriculararrhythmias. The cardiac complexes comprising the rhythm observedfollowing the 215 second mark include both MVT and PVT rhythms. MVTsgenerally correlate poorly with a normal sinus template; however, theserhythms do not have considerable variability between complexes. As such,these rhythms have a low variability score (comparison method D) and arefound in the second region 82 of the graph. In contrast, although PVTsalso correlate poorly with a normal sinus template, they also haveconsiderable variability between succeeding complexes. Thus, theserhythms have a high variability score and would be found populating thethird region 84 of the graph.

For cardiac complexes that resulted in a clear “yes” answer to thisBoolean query (those complexes populating the third region 84 and someportions of the second region 82), the detection enhancement operator 42unambiguously identifies the arrhythmias as PVT or VF. Additionally,because these arrhythmias require therapy, the device would then bedirected to deliver a therapeutic shock following this “yes” answer. Incontrast, a clear “no” answer to this Boolean query (those complexespopulating the first region 80 of the graph) would direct the detectionenhancement operator 42 to withhold therapy based on the comparisons.Finally, an indecisive or weak “no” answer to this Boolean query (thosecomplexes populating some portions of the second region 82) would resultin the detection enhancement operator 12 asking the second layerquestion 77.

FIG. 12 depicts graphically the second layer of questioning 77—doescomparison method D result in a HIGH score and (a Boolean AND) doescomparison method E result in a WIDE score? Again, three distinctregions arise out of the graph in FIG. 12. The first region 86 comprisesthose complexes having a narrow QRS complex and possessing a lowvariability between succeeding cardiac complexes. Rhythms indicative ofthese characteristics are supraventricular in origin and generallycorrespond to normal sinus. In contrast, ventricular originating rhythms(MVT, PVT and VF) possess a wide QRS complex. In addition to possessinga wide QRS, MVTs also have a low variability between successivecomplexes, and therefore, populate the second region 88 in the graph.Similarly, the PVTs and VFs demonstrate a high variability betweensuccessive complexes and therefore populate the third region 90 in thegraph.

A three dimensional representation of both the first and the secondlayer of questioning 75, 77 is depicted in FIG. 13. Comparison methodsA, D and E align the three axes of the graph. When the detectionenhancement operator 42 evaluates the first and second layer ofquestioning 75, 77 on the sample electrocardiogram, a distinct patternarises. Specifically, the supraventricular cardiac complexes 92 (normalsinus rhythms) clearly segregate themselves from the remainingventricular originating complexes 94. Moreover, as indicated above, theresult of the first and second layer of questioning 75, 77 enables thedetection enhancement operator 42 to withhold therapy based on thecomparisons, deliver therapy based on the comparisons, or hold ordeliver therapy based on the comparisons. In the present example, thedetection enhancement operator 42 would withhold therapy on thosecomplexes in the supraventricular region 92 of graph and deliver therapyto those complexes in the ventricular region 94.

FIGS. 14 through 19 depict other illustrative detection enhancementembodiments of the present invention using cascading and the BooleanANDing of comparison methods. Moreover, FIGS. 16, 17, 18 and 19 showembodiments of the present invention that include a third layer ofquestioning for enhancing specificity when discriminating betweenarrhythmias, and ultimately for directing therapy.

Turning to FIG. 14, the illustrative method senses the cardiac rate anddetermines whether the cardiac rate is within the RTEZ 44. If so, thedetection enhancement operator 42 is engaged, and a first layer ofdetermination is whether A=LOW and B=HIGH, as shown at 100. If so, thesystem charges and delivers therapy. If not, the detection enhancementoperator 42 makes a second layer determination of whether A=LOW andF=HIGH, as shown at 102. Again, if so, the system charges and deliverstherapy; if both queries 100, 102 yield no results, the system goes backto sensing the cardiac rate.

Turning to FIG. 15, the illustrative method senses the cardiac rate anddetermines whether the cardiac rate is within the RTEZ 44. If so, thedetection enhancement operator 42 is engaged, and a first layer ofdetermination is whether A=LOW and D=HIGH, as shown at 104. If so,therapy is delivered. Otherwise, the detection enhancement operator 42makes a second layer determination of whether A=LOW and F=HIGH, as shownat 106. If so, therapy is delivered. If both queries 104, 106 fail, thesystem returns to sensing the cardiac rate.

Turning to FIG. 16, the illustrative method senses the cardiac rate anddetermines whether the cardiac rate is within the RTEZ 44. If so, thedetection enhancement operator 42 is engaged, and a first layer ofdetermination is whether A=LOW and D=HIGH, as shown at 108. If so,therapy is delivered. Otherwise, the detection enhancement operator 42makes a second layer determination of whether A=LOW and D=LOW, as shownat 110. If so, therapy is delivered. Otherwise, the detectionenhancement operator 42 makes a third layer determination of whetherA=LOW and E=WIDE, as shown at 112. If so, therapy is delivered. If allthree queries 108, 110, 112 fail, the system returns to sensing thecardiac rate.

Turning to FIG. 17, the illustrative method senses the cardiac rate anddetermines whether the cardiac rate is within the RTEZ 44. If so, thedetection enhancement operator 42 is engaged, and a first layer ofdetermination is whether A=LOW and B=HIGH, as shown at 114. If so,therapy is delivered. Otherwise, the detection enhancement operator 42makes a second layer determination of whether A=LOW and B=LOW, as shownat 116. If so, therapy is delivered. Otherwise, the detectionenhancement operator 42 makes a third layer determination of whetherA=LOW and D=HIGH, as shown at 118. If so, therapy is delivered. If allthree queries 114, 116, 118 fail, the system returns to sensing thecardiac rate.

Turning now to FIG. 18, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether A=LOW and D=HIGH, as shown at 120. If so,therapy is delivered. Otherwise, the detection enhancement operator 42makes a second layer determination of whether A=LOW, as shown at 122. Ifso, therapy is delivered. Otherwise, the detection enhancement operator42 makes a further third layer determination of whether E=WIDE, as shownat 124. If so, therapy is delivered. If all three queries 120, 122, 124fail, the system returns to sensing the cardiac rate.

Turning now to FIG. 19, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether A=LOW and B=HIGH, as shown at 126. If so, thentherapy is delivered. Otherwise, the detection enhancement operator 42makes a second layer determination of whether A=LOW, as shown at 128. Ifso, therapy is delivered. Otherwise, the detection enhancement operator42 makes a further third layer determination of whether E=WIDE, as shownat 130. If so, therapy is delivered. If all three queries 126, 128, 130fail, the system returns to sensing the cardiac rate.

FIG. 20 through FIG. 29 show additional illustrative detectionenhancement embodiments using cascading non-Boolean comparison methods.

Turning now to FIG. 20, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether B=HIGH, as shown at 132. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether A=LOW, as shown at 134. If so,therapy is delivered. If both queries 132, 134 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 21, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether C=LOW, as shown at 136. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether E=WIDE, as shown at 138. If so,therapy is delivered. If both queries 136, 138 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 22, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether C=LOW, as shown at 140. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether A=LOW, as shown at 142. If so,therapy is delivered. If both queries 140, 142 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 23, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether D=HIGH, as shown at 144. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether E=WIDE, as shown at 146. If so,therapy is delivered. If both queries 144, 146 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 24, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether D=HIGH, as shown at 148. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether A=LOW, as shown at 150. If so,therapy is delivered. If both queries 148, 150 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 25, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether F=LOW, as shown at 152. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether E=WIDE, as shown at 154. If so,therapy is delivered. If both queries 152, 154 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 26, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether F=LOW, as shown at 156. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether A=LOW, as shown at 158. If so,therapy is delivered. If both queries 156, 158 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 27, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether A=LOW, as shown at 160. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether E=WIDE, as shown at 162. If so,therapy is delivered. If both queries 160, 162 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 28, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether F=HIGH, as shown at 164. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether E=WIDE, as shown at 166. If so,therapy is delivered. If both queries 164, 166 fail, the system returnsto sensing the cardiac rate.

Turning now to FIG. 29, the illustrative method senses the cardiac rateand determines whether the cardiac rate is within the RTEZ 44. If so,the detection enhancement operator 42 is engaged and makes a first layerdetermination of whether B=HIGH, as shown at 168. If so, therapy isdelivered. Otherwise, the detection enhancement operator 42 makes asecond layer determination of whether E=WIDE, as shown at 170. If so,therapy is delivered. If both queries 168, 170 fail, the system returnsto sensing the cardiac rate.

FIGS. 9 and 14-29 are illustrative examples only; other combinationsusing the results of Tables 1-3, not specifically identified by thesefigures, are additionally possible and are contemplated by the presentinvention.

In addition to cascading one-sided algorithms, ambiguities are alsogreatly reduced by using a subcutaneous electrode ICD system like theone described above with reference to FIG. 1A, rather than a transvenoussystem as shown in FIG. 1B. More specifically, ambiguity is reduced in asubcutaneous electrode ICD system because the subcutaneous system isoptimized physically and spatially for the collection of far fieldvector information. The spatial distance between electrodes (e.g.,between the first sensing electrode 20 and the canister sensingelectrode 16—vector view 1) may enhance visibility of far field vectorinformation. As such, attributes specific to ventricular arrhythmiassuch as polarity changes or other morphological attributes are readilyrecognized in subcutaneous ICD systems. This enhanced taxonomy, in turn,affects the propagation of the data assessed using a particularcomparison method. Table 4 shows some comparison methods that tease outparticular arrhythmias and their corresponding ambiguity percentageusing a subcutaneous ICD system. In particular, Table 4 illustrates howthe ambiguities percentages using certain comparison methods are reducedusing a subcutaneous ICD system.

TABLE 4 AF AT/ST MVT PVT VF A — — LOW (8%) — — B — — — — — C — — HIGH(8%) — — D — — — — — E NARROW NARROW — — — (13%) (13%) F — — — — — G NO— — YES YES (8%) (8%) (8%)

Ambiguity can be further eliminated from these comparison methods byobserving the same comparison method from additional vector views usinga subcutaneous ICD system. As described in detail above, the detectionenhancement operator 12 can mathematically compare acquired cardiaccomplexes (or there vector representations) from two views to theircorresponding template views. This configuration enhances the detectionenhancement operator's ability to discern supraventricular basedarrhythmias from ventricular based arrhythmias. More specifically, it isextremely unlikely that a ventricular based arrhythmia would appear thesame as its stored sinus template in both views. In such an instance, atleast one of the two views would indicate a morphology change, based onits origination in the ventricle, when compared to the stored sinustemplates. Thus, although there may not be a discriminating differencein one view between a ventricular based arrhythmia and a stored sinustemplate, by examining a second view, the distinction would more likelybe pronounced.

By coupling the optimized far field vector sensing of a subcutaneouselectrode ICD system with the ability to sense in multiple views, thecombination reduces virtually all lingering ambiguity when using aparticular comparison method. Moreover, through this combination, onlytwo comparison methods are necessary to discern the supraventricularbased arrhythmias from ventricular based arrhythmias. Table 5 shows theresulting comparison methods that tease out particular arrhythmias andtheir corresponding ambiguity percentage using a subcutaneous ICD systemwith multiple views. In particular, Table 5 illustrates how theambiguities percentages are nearly unappreciable using the twocomparison methods in a subcutaneous ICD system with multiple views.

TABLE 5 AF AT/ST MVT PVT VF A HIGH HIGH LOW LOW LOW (0.1%) (0.1%) (0.3%)(0.05%) (0.05%) D LOW LOW LOW HIGH HIGH (0.1%) (0.1%) (0.2%) (0.05%)(0.05%)

In analyzing Table 5, it is surmised that Boolean ANDing comparisonmethod A with comparison method D permits the detection enhancementoperator 12 to remove all statistically significant ambiguity from itsdecision-making process. In particular, ambiguity is virtuallyeliminated by having the detection enhancement operator evaluate thefollowing—does comparison method A result in a LOW score and (a BooleanAND) does comparison method D result in a HIGH score? A “yes” answer tothis Boolean query unambiguously identifies the arrhythmias PVT and VF.Moreover, PVT and VF demonstrate ambiguities of fractional percentagesfor this evaluation. Additionally, because these arrhythmias requiretherapy, the device would then be directed to deliver a therapeuticshock following this “yes” answer. A “no” answer to this Boolean query,however, would result in the detection enhancement operator withholdingtreatment.

FIGS. 30 and 31 illustrate how the detection enhancement operator 42 mayadditionally distinguish supraventricular arrhythmias from normal sinusrhythms and ventricular arrhythmias. A supraventricular arrhythmiasegment 192 and a normal sinus segment 190 are shown in theelectrocardiogram of FIG. 30. Furthermore, the electrocardiogramillustrates that if rate was the only determinative factor in decidingwhether to apply or withhold therapy, a patient experiencing such asupraventricular arrhythmia would have been delivered an inappropriateshock. The point in the electrocardiogram where an event is declaredusing an industrial standard rate based algorithm is shown as lines 194and 196.

Several embodiments of the present invention greatly reduce theinstances of inappropriate shocks such as the one delivered in FIG. 30.For example, a three dimensional representation of both the first andthe second layer of questioning 75, 77 of FIG. 9 is depicted in FIG. 31.Comparison methods A, D and E align the three axes of the graph. Whenthe detection enhancement operator 42 evaluates the first and secondlayer of questioning 75, 77 on the sample electrocardiogram of FIG. 30,a distinct pattern arises. Specifically, both the complexes of thesupraventricular arrhythmia segment 190 and the normal sinus rhythmsegment 194 populate the same portion of the graph—region 198. Only afew complexes fail to populate this region 198. Moreover, none of thesecomplexes would be capable of initiating therapy because the detectionarchitecture operator 42 further requires an X out of Y filter, which afew stray complexes could not trigger. Thus, in striking comparison toan industry standard rate based algorithm, the illustrative embodimentwould not have delivered therapy based on the comparisons performed bythe detection enhancement operator 42.

The detection enhancement operator of the present invention possessestremendous flexibility. The detection enhancement operator 42 candiscriminate and detect arrhythmias by using comparison methods (A-G)singly (e.g., A alone), in combination with multiple comparison methods(e.g., A with D), in concert with other parameters (e.g., A with rateabove 180 bpm), or in any combination thereof, to direct appropriatetherapy in treating arrhythmias. As a result of this flexibility, thetiming associated with applying appropriate therapy may be a function ofthe rhythm identified and the malignancy of the identified rhythm.

Certain arrhythmias, like ventricular fibrillation, will be identifiedquickly by the detection enhancement operator 42. If these arrhythmiasare ones that require therapy, the detection enhancement operator 42,depending on the device requirements, may deliver therapy quickly. Forexample, the detection enhancement operator 42 may begin charging fortherapy delivery within approximately twenty-four beats after sensingthe first malignant cardiac complex.

Alternatively, other arrhythmias require greater assessment. Thedetection enhancement operator 42 may evaluate multiple comparisonmethods, comparison methods in cascading fashion, different vectorviews, or a combination thereof prior to discerning a particulararrhythmia. For these more complicated cardiac complexes, the detectionenhancement operator 42 is capable of evaluating when to begin preparingfor therapy delivery based on the malignancy of the arrhythmia it isdiscriminating between. If the malignant nature of the arrhythmia beingdiscriminated between is high, the detection enhancement operator 42 maybegin charging for therapy delivery before finally assessing thearrhythmia. If, however, the detection enhancement operator 42 perceivesthe arrhythmia being assessed is most likely a supraventricular event,i.e., a non-life threatening rhythm, the detection enhancement operator42 may withhold therapy delivery until an assessment is finallydetermined.

For the majority of rhythms occurring in patients receiving this type ofdevice, the detection enhancement operator 42 of the present inventionis capable of assessing and treating a life threatening arrhythmiaquickly. For the remainder of the rhythm disorders, the detectionarchitecture of the present invention will take additional time to runthrough the various comparison methods and cascades in order to enhancespecificity. This, in fact, makes clinical sense; where the rapidity andaggressiveness of the device intervention matches the malignancy of thearrhythmia.

The present invention, in some embodiments, is also embodied byoperational circuitry including select electrical components providedwithin the canister 12 (FIG. 1A) or canister 32 (FIG. 1B). In suchembodiments, the operational circuitry may be configured to enable theabove methods to be performed. In some similar embodiments, the presentinvention may be embodied in readable instruction sets such as a programencoded in machine or controller readable media, wherein the readableinstruction sets are provided to enable the operational circuitry toperform the analysis discussed in the above embodiments. Furtherembodiments may include a controller or microcontroller adapted to readand execute the above methods. These various embodiments may incorporatethe illustrative methods shown in FIGS. 9 and 14-29, for example.

The following illustrative embodiments are explained in terms ofoperational circuitry. The operational circuitry may be configured toinclude such controllers, microcontrollers, logic devices, memory, andthe like, as selected, needed, or desired, for performing the methodsteps for which each is adapted.

An illustrative embodiment may comprise an ICD comprising a leadelectrode assembly including a number of electrodes, and a canisterhousing operational circuitry; wherein the lead electrode assembly iscoupled to the canister and the operational circuitry is configured toperform a method for discriminating between arrhythmias which areappropriate for therapy. In the illustrative embodiment, the methodcomprises receiving a cardiac complex using implanted electrodes,obtaining a cardiac rate, determining whether the cardiac rate eitherexceeds a first threshold but does not exceed a second threshold, orexceeds the second threshold; and, if the cardiac rate exceeds thesecond threshold, directing therapy to the heart; or if the cardiac rateexceeds the first threshold but does not exceed the second threshold,directing further analysis of the cardiac complex to determine whethertherapy is indicated. In some related embodiments, the further analysisincludes comparison of the cardiac complex to a template. For one suchrelated embodiment, the comparison includes a correlation waveformanalysis. In another related embodiment, the template is formed byaveraging a number of recent cardiac complexes. In yet another relatedembodiment, the template is a static template. The further analysis mayalso include a determination of a correlation between the cardiaccomplex and a template and comparison of the correlation for the cardiaccomplex to correlations for a number of recent cardiac complexes. Also,the further analysis may include a QRS complex width measurement, adetermination of whether the cardiac rate accelerated significantly, ora determination of the interval rate stability between cardiaccomplexes.

Yet another embodiment includes an ICD comprising a lead electrodeassembly including a number of electrodes and a canister housingoperational circuitry, wherein the lead electrode assembly is coupled tothe canister and the operational circuitry is configured to perform amethod of cardiac analysis. For the illustrative embodiment, the methodmay include receiving a cardiac complex from an implanted electrodepair, analyzing the cardiac complex to determine whether a patient islikely experiencing an arrhythmia, and comparing a portion of thecardiac complex to a template by performing a mathematical calculationbetween the cardiac complex and the template, wherein the comparing stepis performed only if it is determined that the patient is likelyexperiencing an arrhythmia. In related embodiments, the step ofanalyzing the cardiac complex to determine whether an arrhythmia likelyincludes estimating a cardiac rate and comparing the cardiac rate to athreshold value. Some embodiments may also include updating the templateusing data from the cardiac complex. The mathematical calculation mayinclude a correlation waveform analysis. In a further embodiment, thestep of receiving a cardiac complex from an implanted electrode pairincludes receiving a first electrical signal from a first combination ofelectrodes, receiving a second electrical signal from a secondcombination of electrodes, comparing the first electrical signal to thesecond electrical signal to determine which electrical signal is moreamenable to data analysis, and using the electrical signal that is moreamenable to data analysis as the cardiac complex for comparison with thetemplate. In another embodiment, the device may further execute a methodstep including selecting a template for use in the comparison step inresponse to an observed event occurring prior to the receipt of thecardiac complex. Such embodiments may observe and/or treat a monomorphicventricular tachycardia, a polymorphic ventricular tachycardia, orventricular fibrillation.

An illustrative embodiment includes an ICD comprising a lead electrodeassembly including a number of electrodes and a canister housingoperational circuitry, wherein the lead electrode assembly is coupled tothe canister and the operational circuitry is configured to perform amethod of discriminating between cardiac arrhythmias. The method theoperational circuitry is configured to perform may include receiving afirst electric signal between a first electrode pair, analyzing thefirst electric signal to calculate a cardiac rate for a patient,comparing the cardiac rate to first and second thresholds, and selectingone of the following options: a) if the cardiac rate is below the firstthreshold, advancing to a next iteration of the method by receiving asecond electric signal between the first electrode pair, the secondelectric signal coming temporally after the first electric signal; or b)if the cardiac rate is above the second threshold, determining thattherapy should be delivered to the patient; or c) advancing into asubroutine for enhanced analysis, wherein the subroutine for enhancedanalysis includes comparing a portion of the first electric signal to atemplate.

Yet another embodiment includes an ICD comprising a lead electrodeassembly including a number of electrodes and a canister housingoperational circuitry, wherein the lead electrode assembly is coupled tothe canister and the operational circuitry is configured to perform amethod of discriminating between cardiac rhythms comprising receiving acardiac complex, determining that an arrhythmia is likely, analyzing thecardiac complex using a first metric to determine whether a malignantarrhythmia is occurring and, if so, determining that therapy isindicated, if not, then analyzing the cardiac complex using a secondmetric to determine whether a malignant arrhythmia is occurring and, ifso, determining that treatment is indicated. In a further embodiment,the operational circuitry is configured such that both the first metricand the second metric are calculated using the cardiac complex, whereinthe cardiac complex is captured using two electrodes.

Another embodiment includes an ICD comprising a lead electrode assemblyincluding a number of electrodes and a canister housing operationalcircuitry, wherein the lead electrode assembly is coupled to thecanister and the operational circuitry is configured to perform a methodof signal analysis comprising receiving a first cardiac complex from afirst implanted electrode pair disposed to capture electricalinformation related to ventricular activity along a first sensingvector, receiving a second cardiac complex from a second implantedelectrode pair disposed to capture electrical information related toventricular activity along a second sensing vector, generating a firstmetric related to the first cardiac complex, generating a second metricrelated to the second cardiac complex, and comparing the first metric tothe second metric to determine whether a ventricular originatingarrhythmia is occurring. In further embodiments, the first cardiaccomplex and the second cardiac complex are substantially temporallyrelated, the first sensing vector and the second sensing vector areplaced at an angle of greater than 45 degrees with respect to oneanother, the first electrode pair includes first and second electrodes,and the second electrode pair includes the second electrode and a thirdelectrode, and/or the first electrode pair and the second electrode pairare disposed to capture far-field signals for atrial and ventricularevents.

Another embodiment includes an ICD comprising a lead electrode assemblyincluding a number of electrodes and a canister housing operationalcircuitry, wherein the lead electrode assembly is coupled to thecanister and the operational circuitry is configured to perform a methodof monitoring cardiac function as part of the operation of animplantable cardiac treatment device. For the illustrative embodiment,the operational circuitry may be configured to perform a methodincluding receiving a cardiac complex from first and second implantedelectrodes, comparing the cardiac complex to a template to determinewhether therapy is indicated, wherein the template is a dynamicallychanging template formed using a number of recently sensed cardiaccomplexes. In a further embodiment, the step of comparing the cardiaccomplex to a template includes performing a correlation waveformanalysis to generate a correlation coefficient, and comparing thecorrelation coefficient to a threshold.

Another embodiment includes an ICD comprising a lead electrode assemblyincluding a number of electrodes and a canister housing operationalcircuitry, wherein the lead electrode assembly is coupled to thecanister and the operational circuitry is configured to perform a methodof discriminating between cardiac rhythms comprising receiving a cardiaccomplex from implanted electrodes, obtaining a cardiac rate anddetermining whether an arrhythmia is likely; and, if so: (a) analyzingthe cardiac complex using a first mathematical determination to yield afirst result, and comparing the first result to a first threshold toyield a first Boolean value; (b) analyzing the cardiac complex using asecond mathematical determination to yield a second result, andcomparing the second result to a second threshold to yield a secondBoolean value; and (c) performing a first Boolean logic function usingat least one of the first Boolean value and the second Boolean value todetermine whether therapy is needed. In a further embodiment, theoperational circuitry is configured such that the first mathematicaldetermination is a correlation between a static template and the cardiaccomplex, the second mathematical determination is a variability ofcorrelations of several recent cardiac complexes compared to a dynamictemplate, the Boolean logic function observes whether the first Booleanvalue is zero and the second Boolean value is one, and, if the Booleanlogic function yields a one, it is determined that therapy is needed.For another embodiment, the operational circuitry is configured so thefirst mathematical determination is a correlation between a statictemplate and the cardiac complex, the second mathematical determinationis a variability of correlations of several recent cardiac complexescompared to a static template, the Boolean logic function observeswhether the first Boolean value is zero and the second Boolean value isone, and, if the Boolean logic function yields a one, it is determinedthat therapy is needed.

In another embodiment using such Boolean logic, the operationalcircuitry is further configured such that the first mathematicaldetermination is a correlation between a static template and the cardiaccomplex, the second mathematical determination is an analysis of aninterval rate stability for a number of recent cardiac complexes, theBoolean logic function observes whether the first Boolean value is zeroand the second Boolean value is one, and, if the Boolean logic functionyields a one, it is determined that therapy is needed. An illustrativeembodiment includes operational circuitry configured so that the firstmathematical determination is a variability of correlations of severalrecent cardiac complexes compared to a dynamic template, the secondmathematical determination is an analysis of the width of the cardiaccomplex, the Boolean logic function observes whether the first Booleanvalue is one and the second Boolean value is one, and, if the Booleanlogic function yields a one, it is determined that therapy is needed.Yet another embodiment executes a method wherein the first mathematicaldetermination is a correlation between a static template and the cardiaccomplex, the second mathematical determination is a variability ofcorrelations of several recent cardiac complexes compared to a statictemplate, the Boolean logic function observes whether the first Booleanvalue is zero and the second Boolean value is zero, and, if the Booleanlogic function yields a one, it is determined that therapy is needed.

Yet another embodiment using the noted Boolean logic includesoperational circuitry further configured such that the method includesanalyzing the cardiac complex using a third mathematical determinationto yield a third result, and comparing the third result to a thirdthreshold to yield a third Boolean value, and performing a secondBoolean logic function using at least one of the first, second, and/orthird Boolean values to determine whether therapy is needed.

Another embodiment includes an ICD comprising a lead electrode assemblyincluding a number of electrodes and a canister housing operationalcircuitry, wherein the lead electrode assembly is coupled to thecanister and the operational circuitry is configured to perform a methodof discriminating between cardiac rhythms comprising receiving a cardiaccomplex from implanted electrodes, obtaining a cardiac rate anddetermining whether an arrhythmia is likely; and, if so: (a) analyzingthe cardiac complex using a first metric to determine whether amalignant arrhythmia is occurring and, if so, determining that therapyis indicated; and (b) if not, then analyzing the cardiac complex using asecond metric to determine whether a malignant arrhythmia is occurringand, if so, determining that treatment is indicated. In furtherembodiments, the first metric is a comparison of the cardiac complexwidth to a threshold wherein, if the width is greater than the thresholdvalue it is determined that a malignant arrhythmia is occurring, whereinthe second metric is a correlation between the cardiac complex and atemplate, wherein if the correlation is low then it is determined that amalignant arrhythmia is occurring, wherein the template may be static ordynamic. In another embodiment, the second metric is a comparison of thecorrelation of the cardiac complex and a template to the correlation ofa number of recent cardiac complexes to the template to yield avariability, wherein if the variability is high then it is determinedthat a malignant arrhythmia is occurring. Again, the template may beeither static or dynamic. In another embodiment, the first metric is acomparison of a threshold to a correlation between the cardiac complexand a template wherein, if the correlation is low, then it is determinedthat a malignant arrhythmia is occurring. The template may be static ordynamic. In one embodiment, the first metric is a threshold comparisonwith respect to a correlation to a static template, and the secondmetric is a comparison of a threshold to a correlation between thecardiac complex and a dynamic template wherein, if the correlation islow, then it is determined that a malignant arrhythmia is occurring. Inyet another embodiment, the second metric is a determination of thevariability of the correlation between the cardiac complex and thetemplate to correlations between recent cardiac complexes and thetemplate wherein, if the variability is high, then it is determined thata malignant arrhythmia is occurring.

Numerous characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many aspects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size and arrangement of parts without exceeding the scope of theinvention. The invention's scope is defined, of course, in the languagein which the claims are expressed.

1. A method of observing and analyzing cardiac function, the methodcomprising: capturing electrical cardiac signals using at least firstand second electrodes implanted in a patient; selecting portions of thecardiac signals for analysis including a set of samples each indicativeof a cardiac event; generating a number of scores related to thecorrelation of individual samples to a cardiac template; calculating aset variance metric related to a variance in the correlation scores ofthe set of samples; comparing the set variance metric to a threshold;and if the set variance metric does not exceed the threshold,determining that the patient is likely experiencing a malignant cardiacevent.
 2. A method as in claim 1, further comprising: identifying ascore related to the correlation of a most recent sample to the cardiactemplate; comparing the identified score to a threshold; and if theidentified score does not exceed the threshold, determining that thepatient is likely experiencing a malignant cardiac event.
 3. A method asin claim 1, further comprising observing a cardiac triggering event, thecardiac triggering event being the presence of a cardiac event rate inexcess of a predetermined level, wherein the cardiac template is acardiac template representative of non-malignant cardiac eventsoccurring after a cardiac triggering event.
 4. The method of claim 3,wherein the cardiac template is dynamically updated.
 5. The method ofclaim 1, further comprising comparing an individual score threshold toone of the generated number of scores to determine whether an associatedindividual sample is indicative of a malignant cardiac condition.
 6. Amethod of cardiac signal analysis comprising: repeating the method ofclaim 1 for a number, Y, of temporally related sample sets; anddetermining whether electrical cardiac stimulus is indicated by countinga number of sample sets, X, in which it is determined that the patientis likely experiencing a malignant cardiac event, and comparing X to Y.7. A method of cardiac signal analysis comprising: repeating the methodof claim 2 for Y temporally related sample sets; and determining whetherelectrical cardiac stimulus is indicated by counting a number, X, ofsample sets in which it is determined that the patient is likelyexperiencing a malignant cardiac event.
 8. A method of cardiac signalanalysis comprising: collecting a set of electrical signal samples of apatient's cardiac function from implanted electrodes; comparing a numberof individual signal samples to a cardiac signal template to generate acomparison score related to a correlation between the signal sample andthe cardiac signal template; for a number of individual signal samples,marking the individual signal sample as indicative of a malignantcardiac condition if at least one of the following conditions is met:(a) a subset of signal samples including the individual signal sampleand a number of prior signal samples illustrates a variance ofcomparison scores in excess of a predetermined threshold; and (b) acardiac rate triggering event has been detected, the cardiac ratetriggering event comprising the detection of a cardiac rate in excess ofa predetermined level, and a correlation between the individual signalsample and a post-event template falls below a predetermined level; andobserving whether at least a threshold number of signal samples aremarked as indicative of a malignant cardiac condition within apredetermined number of signal samples.
 9. An implantable cardiacstimulus device comprising: operational circuitry contained within animplantable housing; a plurality of electrodes coupled to theoperational circuitry such that the operational circuitry can capturedcardiac electrical data therefrom when implanted in an implantee;wherein the operational circuitry is configured to perform the followingmethod of determining whether the implantee is in need of therapy: a)capturing electrical cardiac signals from the plurality of electrodes;b) selecting portions of the cardiac signals for analysis including aset of samples each indicative of a cardiac event; c) generating anumber of scores related to the correlation of individual samples to acardiac template; d) calculating a set variance metric related to avariance in the correlation scores of the set of samples; e) comparingthe set variance metric to a threshold; and f) if the set variancemetric does not exceed the threshold, determining that the patient islikely experiencing a malignant cardiac event.
 10. The implantablecardiac stimulus device of claim 9 wherein the operational circuitry isfurther configured to: repeat steps a) through f) for a number, Y, oftemporally related sample sets; determine whether electrical cardiacstimulus is indicated by counting a number of sample sets, X, in whichit is determined that the patient is likely experiencing a malignantcardiac event, and comparing X to Y.
 11. The implantable cardiacstimulus device of claim 9 wherein the operational circuitry isconfigured to: identify a score related to the correlation of a mostrecent sample to the cardiac template; compare the identified score to athreshold; and if the identified score does not exceed the threshold,determining that the patient is likely experiencing a malignant cardiacevent.
 12. The implantable cardiac stimulus device of claim 9 whereinthe operational circuitry is further configured to: repeat the method ofclaim 11 for Y temporally related sample sets; and determine whetherelectrical cardiac stimulus is indicated by counting a number, X, ofsample sets in which it is determined that the patient is likelyexperiencing a malignant cardiac event.
 13. The implantable cardiacstimulus device of claim 9 wherein the operational circuitry is furtherconfigured to observe a cardiac triggering event, the cardiac triggeringevent being the presence of a cardiac event rate in excess of apredetermined level, wherein the cardiac template is a cardiac templaterepresentative of non-malignant cardiac events occurring after a cardiactriggering event.
 14. The implantable cardiac stimulus device of claim13 wherein the operational circuitry is further configured such that thecardiac template is dynamically updated.
 15. The implantable cardiacstimulus device of claim 9 wherein the operational circuitry is furtherconfigured to: compare an individual score threshold to one of thegenerated number of scores to determine whether an associated individualsample is indicative of a malignant cardiac condition.
 16. Animplantable cardiac stimulus device comprising: operational circuitrycontained within an implantable housing; a plurality of electrodescoupled to the operational circuitry such that the operational circuitrycan captured cardiac electrical data therefrom when implanted in animplantee; wherein the operational circuitry is configured to performthe following method of determining whether the implantee is in need oftherapy: collect a set of electrical signal samples of a patient'scardiac function from the plurality of electrodes; compare a number ofindividual signal samples to a cardiac signal template to generate acomparison score related to a correlation between the signal sample andthe cardiac signal template; for a number of individual signal samples,marking the individual signal sample as indicative of a malignantcardiac condition if at least one of the following conditions is met:(a) a subset of signal samples including the individual signal sampleand a number of prior signal samples illustrates a variance ofcomparison scores in excess of a predetermined threshold; and (b) acardiac rate triggering event has been detected, the cardiac ratetriggering event comprising the detection of a cardiac rate in excess ofa predetermined level, and a correlation between the individual signalsample and a post-event template falls below a predetermined level; andobserve whether at least a threshold number of signal samples are markedas indicative of a malignant cardiac condition within a predeterminednumber of signal samples.